Secondary battery comprising insulator and reinforcing filler

ABSTRACT

Disclosed is a secondary battery having a structure in which a jelly-roll having a cathode/separator/anode structure is mounted in a cylindrical battery case and a plate-shaped insulator is mounted on top of the jelly-roll, wherein the insulator has a porous structure in which a plurality of fine pores communicate in a longitudinal direction, or in traverse and longitudinal directions.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Bypass Continuation of PCT InternationalApplication No. PCT/KR2012/007434 filed on Sep. 18, 2012, which claimspriority under 35 U.S.C. §119(a) to Patent Application No.10-2011-0097269 filed in the Republic of Korea on Sep. 27, 2011, all ofwhich are hereby expressly incorporated by reference into the presentapplication.

TECHNICAL FIELD

The present invention relates to a secondary battery with superiorproductivity and safety. More specifically, the present inventionrelates to a secondary battery having a structure in which a jelly-rollhaving a cathode/separator/anode structure is mounted in a cylindricalbattery case and a plate-shaped insulator is mounted on top of thejelly-roll, wherein the insulator has a porous structure in which aplurality of fine pores communicate in a longitudinal direction, or intraverse and longitudinal directions.

BACKGROUND ART

The development of techniques associated with mobile devices andincrease in demand therefor have brought about rapid increase in thedemand for secondary batteries as energy sources. Among secondarybatteries, lithium secondary batteries with high energy density, highdriving voltage and superior storage and lifespan characteristics arewidely used as energy sources of various electric products includingmobile devices.

Depending on the shape of the battery case, the secondary battery may bedivided into cylindrical and rectangular batteries mounted incylindrical and rectangular metal cans, respectively, and a pouch-shapedbattery mounted in a pouch-shaped case made of an aluminum laminatesheet. Of these, the cylindrical battery has advantages of relativelyhigh capacity and superior structural stability. The electrode assemblymounted in the battery case is an electricity-generating device enablingcharge and discharge that has a cathode/separator/anode laminatestructure and is divided into a jelly-roll type in which an electrodeassembly including a separator interposed between a cathode and ananode, each made of an active material-coated long sheet, is rolled, astack-type in which a plurality of cathodes and a plurality of anodesare laminated in this order such that a separator is interposed betweenthe cathode and the anode and a stack/folding type which is acombination of a jelly-roll type and a stack type. Of these, thejelly-roll-type electrode assembly has advantages of easy manufactureand high energy density per unit weight.

In this regard, a conventional cylindrical secondary battery is shown inFIG. 1. An insulator generally used for the cylindrical secondarybattery is shown in plan views of FIGS. 2 and 3.

Referring to FIGS. 2 and 3, a cylindrical secondary battery 100 ismanufactured by mounting a jelly-roll type (rolled-type) electrodeassembly 120 in a battery case 130, injecting an electrolyte into thebattery case 130 and coupling a cap assembly 140 provided with anelectrode terminal (for example, a cathode terminal; not shown) to theopen top of the case 130.

The electrode assembly 120 is obtained by inserting a separator 123between a cathode 121 and an anode 122, and rolling the resultingstructure into a round shape. A cylindrical center pin 150 is insertedinto the core (center) of the jelly-roll. The center pin 150 isgenerally made of a metal to impart a predetermined strength and has ahollow-shaped cylindrical structure of a roundly bent plate material.Such a center pin 150 sets and supports the electrode assembly andserves as a passage, enabling discharge of gas generated by internalreaction during charge and discharge, and operation.

In addition, a plate-shaped insulator 180 a is mounted on top of theelectrode assembly 120, and is provided in the center thereof with anopening 181 a communicating with the through hole 151 of the center pin150 so that gas is discharged and the cathode tap 142 of the electrodeassembly 120 is connected to the cap plate 145 of the cap assembly 140.

However, the insulator 180 a arranged on top of the jelly-roll is astructure that blocks a passage, enabling permeation of electrolyte intoa battery in the process of injecting the electrolyte into the battery.For this reason, the electrolyte permeates the battery only through theopening 181 a communicating with the center pin 150 and a regionexcluding the insulator 180 a, thus disadvantageously requiring a longtime for injection of electrolyte and consequently causing deteriorationin productivity.

In order to improve permeability of the electrolyte, as shown in FIG. 3,a partial connection member 180 b having a structure in which aplurality of through pores 182 b are formed around an opening 181 b issuggested.

However, this structure is found to have serious problems in terms ofsafety. That is, conductive impurity particles such as metal powdersgenerated in the process of manufacturing and/or assembling the capassembly 140, the battery case 130 and the like are permeated into theelectrode assembly 120 through the through pores 182 b that areperforated in the insulator 180 b, thus disadvantageously causingoccurrence of short circuit or deterioration in battery lifespan.

Accordingly, there is an increasing need for secondary batteries thatenhance injection processability of electrolyte and preventincorporation of foreign matter in the process of assembling batteries,thereby improving lifespan.

DISCLOSURE Technical Problem

Therefore, the present invention has been made to solve the above andother technical problems that have yet to be resolved.

As a result of a variety of extensive and intensive studies andexperiments to solve the problems as described above, the presentinventors developed an insulator having a specific shape described belowand discovered that the insulator considerably enhances injectability ofelectrolyte, prevents, in the jelly-roll, incorporation of foreignmatter produced during an assembly process such as beading, and therebyprevents battery defects and improves safety. The present invention hasbeen completed, based on this discovery.

Technical Solution

In accordance with one aspect of the present invention, provided is asecondary battery having a structure in which a jelly-roll having acathode/separator/anode structure is mounted in a cylindrical batterycase and a plate-shaped insulator is mounted on top of the jelly-roll,wherein the insulator has a porous structure in which a plurality offine pores communicate in a longitudinal direction, or in traverse andlongitudinal directions. Accordingly, in the secondary battery accordingto the present invention, an electrolyte is injected through a pluralityof fine pores having a specific shape and, as a result, injection timecan be shortened and injectability is thus improved.

In the porous structure, size, position and number of the fine pores anddistance therebetween are not limited as long as they do not impairprevention of incorporation of foreign matter, electrolyte injectabilityand gas discharge.

Preferably, the fine pores provide electric insulation as an inherentfunction of an insulator, and have high permeability to an electrolyteduring injection of electrolyte and a size of 1 μm to 100 μm in order toprevent permeation of foreign matter having a size of 100 μm or more.

In this case, there is no risk of incorporation of foreign matter intothe jell-roll during electrolyte injection. Thus, advantageously, it ispossible to omit a process of screening and removing foreign matter, toremove a risk of short circuit resulting from incorporation of foreignmatter and thus improve productivity.

In a specific embodiment, the fine pores may be spaced from one anotherover the entire surface of the insulator in order to preventincorporation of foreign matter having a size of 100 μm or more, andimprove injectability of electrolyte and gas discharge.

When an electrolyte is injected into the fine pores formed over theentire surface of the insulator, injection passages may be furtherbranched, injectability is improved, injection time can be reduced,injection speed is constant at a predetermined distance between finepores, the electrolyte can be uniformly impregnated into the jelly-rolland, as a result, battery properties are advantageously improved.

In addition, the fine pores spaced from one another over the entiresurface of the insulator provide passages, enabling discharge of gasgenerated during electrolyte decomposition. In terms of diffusion ofgas, discharge speed may be increased when the gas is discharged throughthe branched discharge passages.

In the present invention, the expression “the fine pores communicate ina longitudinal direction or in longitudinal and traverse directions”means that the fine pores communicate at least in a vertical directionin the plate-shaped insulator. Accordingly, electrolyte injected intothe insulator can move to lower parts of the insulator via the finepores that communicate at least in a longitudinal direction. As definedabove, such fine pores may communicate not only in a longitudinaldirection but also in a traverse direction (in a horizontal direction inthe plate-shaped insulator).

All fine pores do not necessarily communicate, and 50% or more of finepores preferably communicate, and 70% or more of fine pores morepreferably communicate. It may be understood that these fine pores are akind of open-type pores.

Such communicated fine pores may be produced by various methods. Forexample, a chemical or physical foaming agent is added to a melt orsolution for forming a material made of polymer resin or polymercomposite as a material for the insulator, followed by foaming, or asoluble filler is added to the melt or solution and the filler is thenremoved, but the method of forming fine pores is not limited thereto.

In a specific embodiment, the communicated fine pores may be formed bytreatment of a foaming agent. Examples of the physical foaming agentinclude butane, pentane or the like used for products such as expendedpolystyrene (EPS) and EPP. Examples of chemical foaming agents includeinorganic foaming agents such as sodium bicarbonate (NaHCO₃) and organicfoaming agents such as azodicaronamide (ADAC), P,P′-oxybis(benzenesulfonyl hydrazide (OBSH), and p-toluene sulfonyl hydrazine (TSH).

In general, as porosity increases, a density of the insulator decreasesand mechanical properties are deteriorated. For this reason, by adding areinforcing filler to a polymer resin or polymer resin composite,porosity can be maintained and desired mechanical properties can besecured. The reinforcing filler may have a granular or fibrous shape,and the content thereof is not particularly limited. For example, thecontent of the filler is 0.5 to 20% by weight, preferably 1 to 10% byweight, based on the total weight of the insulator.

The insulator may be made of any insulating material without particularlimitation and is for example made of an electrical insulating resin oran electrical insulating polymer composite. Specifically, the polymerresin may be one or more selected from the group consisting ofpolyethylene (PE), polypropylene (PP), polybutylene (PB), polystyrene(PS), polyethylene terephthalate (PET), natural rubbers and syntheticrubbers.

Also, the insulator has a total thickness of 0.1 mm to 0.5 mm. When thethickness of the insulator is excessively small, the inherent electricinsulating function of the insulator may not be sufficiently exerted,and when the thickness thereof is excessively large, disadvantageously,jelly-roll size is decreased and battery capacity is thus decreased inthe battery case having the same size.

Preferably, the insulator includes an opening to enable pass-through ofthe electrode terminals.

Preferably, the secondary battery according to the present invention maybe applied to a lithium secondary battery fabricated by impregnating alithium-containing electrolyte in the jelly-roll.

In general, a lithium secondary battery comprises a cathode, an anode, aseparator, a lithium-containing aqueous electrolyte and the like.

For example, the cathode is produced by applying a slurry prepared bymixing a cathode mixture containing a cathode active material andoptionally containing a conductive material, a binder, a filler and thelike with a solvent such as NMP to a cathode current collector, followedby drying and rolling.

Examples of the cathode active material include, but are not limited to,layered compounds such as lithium cobalt oxide (LiCoO₂) or lithiumnickel oxide (LiNiO₂) or compounds substituted with one or moretransition metals; lithium manganese oxides such as Li_(1+y)Mn_(2−y)O₄(in which y is 0 to 0.33), LiMnO₃ and LiMn₂O₃, and LiMnO₂; lithiumcopper oxides (Li₂CuO₂); vanadium oxides such as LiV₃O₈, LiFe₃O₄, V₂O₅,and Cu₂V₂O₇; Ni site-type lithium nickel oxides represented byLiNi_(1−y)M_(y)O₂ (in which M=Co, Mn, Al, Cu, Fe, Mg, B or Ga, y=0.01 to0.3); lithium manganese composite oxides represented by formula ofLiMn_(2−y)M_(y)O₂ (in which M=Co, Ni, Fe, Cr, Zn or Ta, y=0.01 to 0.1)or Li₂Mn₃M_(y)O₈ (in which, M=Fe, Co, Ni, Cu or Zn); LiMn₂O₄ in which apart of Li is substituted by an alkaline earth metal ion; disulfidecompounds; Fe₂(MoO₄)₃ and the like.

The cathode current collector is generally manufactured to have athickness of 3 to 500 μm. Any cathode current collector may be usedwithout particular limitation so long as it has suitable conductivitywithout causing adverse chemical changes in the manufactured battery.Examples of the cathode current collector include stainless steel,aluminum, nickel, titanium, sintered carbon, and aluminum or stainlesssteel surface-treated with carbon, nickel, titanium or silver. Thesecurrent collectors include fine irregularities on the surface thereof soas to enhance adhesion to electrode active materials. In addition, thecurrent collectors may be used in various forms including films, sheets,foils, nets, porous structures, foams and non-woven fabrics.

The conductive material is commonly added in an amount of 1 to 30% byweight, based on the total weight of the mixture comprising the cathodeactive material. Any conductive material may be used without particularlimitation so long as it has suitable conductivity without causingadverse chemical changes in the battery. Examples of conductivematerials include conductive materials including graphite; carbon blackssuch as carbon black, acetylene black, Ketjen black, channel black,furnace black, lamp black and thermal black; conductive fibers such ascarbon fibers and metallic fibers; metallic powders such as carbonfluoride powders, aluminum powders and nickel powders; conductivewhiskers such as zinc oxide and potassium titanate; conductive metaloxides such as titanium oxide; and polyphenylene derivatives.

The binder is a component which enhances binding of an electrode activematerial to a conductive material and current collector. The binder iscommonly added in an amount of 1 to 30% by weight, based on the totalweight of the mixture comprising the cathode active material. Examplesof the binder include polyvinylidene, polyvinyl alcohol,carboxymethylcellulose (CMC), starch, hydroxypropylcellulose,regenerated cellulose, polyvinyl pyrollidone, tetrafluoroethylene,polyethylene, polypropylene, ethylene propylene diene terpolymer (EPDM),sulfonated EPDM, styrene butadiene rubbers, fluororubbers and variouscopolymers.

The filler is a component optionally used to inhibit expansion of theelectrode. Any filler may be used without particular limitation so longas it does not cause adverse chemical changes in the manufacturedbattery and is a fibrous material. Examples of the filler include olefinpolymers such as polyethylene and polypropylene; and fibrous materialssuch as glass fibers and carbon fibers.

The separator is interposed between the cathode and the anode. As theseparator, an insulating thin film having high ion permeability andmechanical strength is used. The separator typically has a pore diameterof 0.01 to 10 μm and a thickness of 5 to 300 μm. As the separator,sheets or non-woven fabrics made of an olefin polymer such aspolypropylene and/or glass fibers or polyethylene, which have chemicalresistance and hydrophobicity, are used. When a solid electrolyte suchas a polymer is employed as the electrolyte, the solid electrolyte mayalso serve as both the separator and electrolyte.

For example, the anode is produced by applying a slurry prepared bymixing an anode mixture containing an anode active material with asolvent such as NMP to an anode current collector, followed by dryingand rolling. The anode mixture may further optionally contain thecomponents described above.

Examples of the anode active material include carbon such as hardcarbon, graphite-based carbon; metal composite oxides such asLi_(x)Fe₂O₃ (0≦x≦1), Li_(x)WO₂ (0≦x≦1), Sn_(x)Me_(1−x)Me′_(y)O_(z) (Me:Mn, Fe, Pb, Ge; Me′: Al, B, P, Si, Group I, II and III elements,halogen; 0<x≦1; 1≦y≦3; 1≦z≦8); lithium metals; lithium alloys;silicon-based alloys; tin-based alloys; metal oxides such as SnO, SnO₂,PbO, PbO₂, Pb₂O₃, Pb₃O₄, Sb₂O₃, Sb₂O₄, Sb₂O₅, GeO, GeO₂, Bi₂O₃, Bi₂O₄,and Bi₂O₅; conductive polymers such as polyacetylene; Li—Co—Ni-basedmaterials and the like.

The anode current collector is generally fabricated to have a thicknessof 3 to 500 μm. Any anode current collector may be used withoutparticular limitation so long as it has suitable conductivity withoutcausing adverse chemical changes in the manufactured battery. Examplesof the anode current collector include copper, stainless steel,aluminum, nickel, titanium, sintered carbon, and copper or stainlesssteel surface-treated with carbon, nickel, titanium or silver, andaluminum-cadmium alloys. Similar to the cathode current collectors, thecurrent collectors include fine irregularities on the surface thereof soas to enhance adhesion to electrode active materials. In addition, thecurrent collectors may be used in various forms including films, sheets,foils, nets, porous structures, foams and non-woven fabrics.

Meanwhile, the electrolyte is composed of a non-aqueous electrolyte anda lithium salt. Examples of preferred electrolytes include non-aqueousorganic solvents, organic solid electrolytes, inorganic solidelectrolytes and the like.

Examples of the non-aqueous solvent include non-protic organic solventssuch as N-methyl-2-pyrollidinone, propylene carbonate, ethylenecarbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate,gamma-butyrolactone, 1,2-dimethoxy ethane, tetrahydroxy franc, 2-methyltetrahydrofuran, dimethylsulfoxide, 1,3-dioxolane, formamide,dimethylformamide, dioxolane, acetonitrile, nitromethane, methylformate, methyl acetate, phosphoric acid triester, trimethoxy methane,dioxolane derivatives, sulfolane, methyl sulfolane,1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives,tetrahydrofuran derivatives, ether, methyl propionate and ethylpropionate.

Examples of the organic solid electrolyte include polyethylenederivatives, polyethylene oxide derivatives, polypropylene oxidederivatives, phosphoric acid ester polymers, poly agitation lysine,polyester sulfide, polyvinyl alcohols, polyvinylidene fluoride, andpolymers containing ionic dissociation groups.

Examples of the inorganic solid electrolyte include nitrides, halidesand sulfates of lithium such as Li₃N, LiI, Li₅NI₂, Li₃N—LiI—LiOH,LiSiO₄, LiSiO₄—LiI—LiOH, Li₂SiS₃, Li₄SiO₄, Li₄SiO₄—LiI—LiOH andLi₃PO₄—Li₂S—SiS₂.

The lithium salt is a material that is readily soluble in theabove-mentioned non-aqueous electrolyte and examples thereof includeLiCl, LiBr, LiI, LiClO₄, LiBF₄, LiB₁₀Cl₁₀, LiPF₆, LiCF₃SO₃, LiCF₃CO₂,LiAsF₆, LiSbF₆, LiAlCl₄, CH₃SO₃Li, CF₃SO₃Li, (CF₃SO₂)₂NLi, chloroboranelithium, lower aliphatic carboxylic acid lithium, lithium tetraphenylborate and imides.

Additionally, in order to improve charge/discharge characteristics andflame retardancy, for example, pyridine, triethylphosphite,triethanolamine, cyclic ether, ethylenediamine, n-glyme, hexaphosphorictriamide, nitrobenzene derivatives, sulfur, quinone imine dyes,N-substituted oxazolidinone, N,N-substituted imidazolidine, ethyleneglycol dialkyl ether, ammonium salts, pyrrole, 2-methoxy ethanol,aluminum trichloride or the like may be added to the non-aqueouselectrolyte. If necessary, in order to impart incombustibility, thenon-aqueous electrolyte may further include halogen-containing solventssuch as carbon tetrachloride and ethylene trifluoride. Further, in orderto improve high-temperature storage characteristics, the non-aqueouselectrolyte may additionally contain carbon dioxide gas, fluoro-ethylenecarbonate (FEC), propene sultone (PRS) or fluoro-ethylene carbonate(FEC).

The present invention also provides a device comprising the secondarybattery as a power source and the device according to the presentinvention is preferably used for mobile devices such as cellular phonesand portable computers as well as electric bikes (E-bikes), electricvehicles (EVs), hybrid electric vehicles (HEVs), plug-in hybrid electricvehicles and power-storage devices in terms of superior lifespan andsafety.

The structures and fabrication methods of the lithium secondary battery,and medium and large battery modules and devices including the lithiumsecondary battery as a unit battery are well-known in the art and adetailed description thereof is thus omitted.

Effects of Invention

As apparent from the fore-going, the secondary battery according to thepresent invention does not need a process for screening and removingforeign matter and can branch injection passages of electrolyte, thusadvantageously greatly improving productivity.

The secondary battery according to the present invention has no risk ofshort circuit resulting from incorporation of foreign matter andimproves gas exhaust, thus enhancing safety.

Also, the secondary battery according to the present invention improvesrate characteristics since a jelly-roll is evenly impregnated in anelectrolyte.

Also, the secondary battery according to the present invention canmaintain porosity and secure a desired mechanical strength, thusimproving productivity.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and other advantages of thepresent invention will be more clearly understood from the followingdetailed description taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a schematic sectional view illustrating a representativecylindrical secondary battery;

FIG. 2 is a plan view illustrating an insulator used for the secondarybattery of FIG. 1 according to one embodiment;

FIG. 3 is a plan view illustrating an insulator used for the secondarybattery of FIG. 1 according to another embodiment; and

FIG. 4 is a plan view illustrating an insulator according to oneembodiment of the present invention.

BEST MODE

Now, the present invention will be described in more detail withreference to the following examples. These examples are provided onlyfor illustration of the present invention and should not be construed aslimiting the scope and spirit of the present invention.

FIG. 4 is a plan view schematically illustrating an insulator accordingto one embodiment of the present invention.

Referring to FIGS. 4 and 1, a secondary battery 100 has a structure inwhich a jelly-roll 120 having a cathode 121/separator 123/anode 122structure is mounted in a cylindrical battery case 130, wherein aninsulator 180 c is mounted on top of the jelly-roll 120.

The insulator 180 c comprises polyethylene terephthalate (PET) with athickness of about 0.4 mm, is provided at one side thereof with anopening 181 c and is provided over the entire surface thereof with aplurality of fine pores 182 c having a diameter of 10 to 30 μm spacedfrom one another by a predetermined distance.

Accordingly, an electrolyte permeates through the plurality of finepores 182 c into the entire surface of the insulator 180 c after beinginjected, thus causing considerable improvement in injectability andpreventing short circuit.

Now, the present invention will be described in more detail withreference to the following examples. These examples are provided only toillustrate the present invention and should not be construed as limitingthe scope and spirit of the present invention.

Example 1

An insulator having a thickness of 0.4 mm, including a rectangularopening with a width of 6 mm and a length of 2.5 mm perforated in oneside thereof and including a plurality of fine pores having a diameterof about 10 μm to 30 μm uniformly distributed by a predetermineddistance of about 10 μm to 30 μm over the entire surface thereof wasmanufactured using PET as a material and ADAC as a foaming agent, asshown in FIG. 4. Then, the insulator was mounted on top of a jelly-rollin which a cathode/separator/anode structure is rolled based on a centerpin and a cylindrical secondary battery of 18650 standard (diameter 18mm, length 65 mm) was manufactured in a state that fine metal powdersgenerated in the process of battery assembly were positioned on theinsulator.

Example 2

An insulator and a secondary battery were manufactured in the samemanner as in Example 1 except that an insulator including a plurality offine pores having a diameter of 100 μm uniformly distributed by apredetermined distance of about 120 μm over the entire surface thereofwas prepared.

Comparative Example 1

An insulator and a secondary battery were manufactured in the samemanner as in Example 1 except that a plurality of fine pores were notincluded, as shown in FIG. 2.

Comparative Example 2

An insulator and a secondary battery were manufactured in the samemanner as in Example 1 except that three through holes having a diameterof 2.5 mm were formed, instead of the fine pores, as shown in FIG. 3.

Comparative Example 3

An insulator and secondary battery were manufactured in the same manneras in Example 1 except that an insulator including a plurality of finepores having a diameter of 150 μm uniformly distributed by apredetermined distance of about 120 μm over the entire surface thereofwas prepared.

Test Example 1

The secondary batteries fabricated in Examples 1 and 2 and ComparativeExamples 1 to 3 were subjected to electrolyte impregnation testing. Theresults are shown in Table 1 below. Electrolyte impregnation testing wascarried out by injecting a 1M LiPF₆ carbonate electrolyte into themanufactured cylindrical battery case, measuring a time taken untilimpregnation ratio of the jelly-roll reached 100%, repeating thisprocess four times and calculating an average of the four values.

In addition, a cap assembly was welded to the open top of the fabricatedsecondary battery to produce 10 samples. The samples were subjected tocharge and discharge testing and generation of short circuit wasconfirmed. Results are shown in Table 1 below.

TABLE 1 Time shortage Number of short- Short Impregnation ratio (%)circuited circuit time (based on Comp. batteries ratio (sec) Ex. 1) (n)(%) Ex. 1 335 56 0 0 Ex. 2 329 55 0 0 Comp. 698 0 2 20 Ex. 1 Comp. 53823 4 40 Ex. 2 Comp. 301 57 1 10 Ex. 3

As can be seen from Table 1, the batteries of Examples 1 to 2 accordingto the present invention had considerably shortened electrolyteimpregnation time, as compared to Comparative Example 1. That is, itcould be seen that the electrolyte was effectively permeated through aplurality of fine pores formed in the insulator.

The battery of Comparative Example 2 exhibited improved impregnation,but increased short circuit, as compared to the battery of ComparativeExample 1, while the battery of Comparative Example 3 also exhibitedimpregnation comparable to Examples 1 and 2, but exhibited higher shortcircuit rate. The reason for this was found to be that metal powderswere permeated into relatively large pores, causing short circuit in thejelly-roll.

On the other hand, the battery of Comparative Example 1 exhibited highshort circuit rates as compared to the batteries of Examples 1 and 2,although fine pores were not perforated in the insulator mounted on thebattery. The reason for the high short circuit rate was believed to bedue to the fact that, in the batteries of Examples 1 and 2, movement ofmetal powders was suppressed when metal powders were entrapped in thefine pores, while, in the battery of Comparative Example 1, metalpowders freely moved on the smooth surface of the insulator and thusmoved to the jelly-roll through the circumference of the opening orinsulator.

Although the preferred embodiments of the present invention have beendisclosed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions arepossible, without departing from the scope and spirit of the inventionas disclosed in the accompanying claims.

The invention claimed is:
 1. A secondary battery having a structure inwhich a jelly-roll having a cathode/separator/anode structure is mountedin a cylindrical battery case and a plate-shaped insulator is mounted ontop of the jelly-roll, wherein the insulator has a porous structure inwhich a plurality of pores communicate in a longitudinal direction, orin traverse and longitudinal directions, and further comprises areinforcing filler to improve mechanical strength, and wherein thereinforcing filler has a granular or fibrous shape.
 2. The secondarybattery according to claim 1, wherein the pores are formed over theentire surface of the insulator.
 3. The secondary battery according toclaim 1, wherein the insulator comprises an electrical insulatingpolymer resin or an electrical insulating polymer composite.
 4. Thesecondary battery according to claim 3, wherein the polymer resin is atleast one selected from the group consisting of polyethylene (PE),polypropylene (PP), polybutylene (PB), polystyrene (PS), polyethyleneterephthalate (PET), natural rubbers and synthetic rubbers.
 5. Thesecondary battery according to claim 1, wherein the insulator has athickness of 0.1 mm to 0.5 mm.
 6. The secondary battery according toclaim 1, wherein the insulator comprises an opening to enablepass-through of electrode terminals.
 7. The secondary battery accordingto claim 1, wherein the battery is a lithium secondary battery.
 8. Adevice comprising the secondary battery according to claim 1 as a powersource.
 9. The device according to claim 8, wherein the device isselected from a cellular phone, a portable computer, an electric bike(E-bike), an electric vehicle, a hybrid electric vehicle, a plug-inhybrid electric vehicle and a power-storage device.
 10. The secondarybattery according to claim 1, wherein the pores have a size of 1 μm to100 μm.